The chameleon on the branches: spectral state transition and dips in NGC 247 ULX-1

TL;DR

This study investigates spectral state transitions and dipping phenomena in the ULX NGC 247 ULX-1 through extensive X-ray observations, revealing thermal components and proposing geometrical or propeller-based explanations.

Contribution

First detailed spectral analysis of NGC 247 ULX-1's state transitions, identifying thermal components and exploring potential mechanisms behind dips and spectral changes.

Findings

Two thermal components describe the spectra: a cold wind emission and a hotter disk.

Spectral evolution suggests possible geometrical occultation or propeller effect.

Identified two distinct spectral branches: normal and dipping.

Abstract

Soft Ultra-Luminous X-ray (ULXs) sources are a subclass of the ULXs that can switch from a supersoft spectral state, where most of the luminosity is emitted below 1 keV, to a soft spectral state with significant emission above 1 keV. In a few systems, dips have been observed. The mechanism behind this state transition and the dips nature are still debated. To investigate these issues, we obtained a long XMM-Newton monitoring campaign of a member of this class, NGC 247 ULX-1. We computed the hardness-intensity diagram for the whole dataset and identified two different branches: the normal branch and the dipping branch, which we study with four and three hardness-intensity resolved spectra, respectively. All seven spectra are well described by two thermal components: a colder ($kT_{\rm bb}$ $\sim$ 0.1-0.2 keV) black-body, interpreted as emission from the photo-sphere of a radiatively-driven wind, and a hotter ($kT_{\rm disk}$ $\sim$ 0.6 keV) multicolour disk black-body, likely due to reprocessing of radiation emitted from the innermost regions. In addition, a complex pattern of emission and absorption lines has been taken into account based on previous high-resolution spectroscopic results. We studied the evolution of spectral parameters and the flux of the two thermal components along the two branches and discuss two scenarios possibly connecting the state transition and the dipping phenomenon. One is based on geometrical occultation of the emitting regions, the other invokes the onset of a propeller effect.